The invention relates to an optical fiber for use in making various fiber devices, such as long-period gratings, of optical communication systems, and deals more particularly with an improved fiber and fiber devices having improved performance in response to changes in temperature. The invention also relates to methods for making such optical fiber and fiber devices.
In the development of fiber optic communication systems it has been found possible and often desirable to form certain circuit components or devices with the fibers themselves. Known in the art are amplifiers, couplers, lasers and filters formed with optical fibers. For example, a filter can be formed by introducing a periodic structure, such as a grating to the fiber. It is known to form a filter, such as, for example, a Bragg grating filter, by controllably varying the index of refraction of the core, and also possibly the index of refraction of the cladding layer immediately surrounding the core, along the length of the fiber. In particular, photosensitive fibers are known wherein at least the core of the fiber includes dopant materials allowing the initial or base index of refraction of the core to be changed from its base value to some other value by the application of actinic radiation to the fiber, and it is also known to form a Bragg filter or grating in such a photosensitive fiber by changing the index of refraction of the core from its base value at a plurality of spaced points along the length of the core, which points of changed index then reflect or reject light of a wavelength which resonates with the points of changed index. As another example, it is also known to form a filter by periodically thinning the fiber with a microburner, as is described in U.S. Pat. No. 5,708,740, issued on Jan. 13, 1998 and assigned to Northern Telecom Limited.
In the making and use of fiber optic system components from specialty fibers it is known that the components customarily have response characteristics dependent on the temperature at which the components are used.
To obtain fixed responses despite varying environmental temperatures various complex and expensive solutions have been proposed in the past, such as housing the components in temperature controlled containers or adding temperature sensitive mechanical stressing features to the fiber. Other attempts at solving the problem have been directed to the selection and proportioning of the constituents of the core and cladding materials of fibers, used in making the components, whereby components made from the fibers inherently have good temperature insensitivity and can hopefully be used in many applications without the need for temperature controlled containers, stressing mechanisms or the like.
In regard to optical communication fibers with filters as described above, it has been found that such filters made in the past have a peak transmission loss wavelength which varies with temperature, and therefore if the operating temperature of the filter changes significantly the ability of the filter to efficiently reject or pass the wavelength(s) for which it was designed becomes impaired. In certain components or devices this problem can arise because the time required for light of a given wavelength to move from one point to another along the length of the core varies with the temperature, with the result that with changes in temperature the light of the wavelength to be rejected or passed by the filter loses its exact resonance with the spaced points of changed refractivity and causes the points of changed refractivity index to reflect or transmit light of the given wavelength with less efficiency.
Prior efforts toward providing an optical fiber with high temperature insensitivity by core and cladding constituent control are revealed by U.S. Pat. Nos. 5,703,978, 5,949,934 and 6,201,918. These patents suggest that changes with respect to temperature of a long-period grating filter made from a given fiber are directly related to changes in the fiber""s effective refractive index neff with respect to temperature. These patents still further suggest that a fiber with high temperature insensitivity can be made by formulating the constituents of the core and cladding materials such that the characteristic curve of the core index vs. temperature and the characteristic curve of the cladding index with respect to temperature have substantially the same shape (see FIG. 5 of the ""978 patent) so that at every reasonable temperature the rate of change of the core index,             ⅆ              n        core                    ⅆ      T        ,
is equal to the rate of change of the cladding index,             ⅆ              n        clad                    ⅆ      T        ,
to accordingly make the rate of change of the fiber effective index acceptably low or zero at all temperatures.
In regard to the disclosures of the foregoing patents, applicants have found that it is impractical, and perhaps impossible, to in all cases formulate core and cladding materials such that a grating or other device made with the fiber has the requisite temperature performance.
Accordingly, it is an object of the present invention to address one or more of the foregoing disadvantages or deficiencies of the prior art.
In one aspect, the invention resides in the finding by applicants that for a fiber having a core co-doped with selected constituents, such as, for example, germanium and boron, the temperature sensitivity characteristic of a long period grating formed in the fiber core, as expressed by a plot of the absolute change in grating wavelength vs. temperature, is a characteristic curve having a temperature Tm at which the sensitivity of the grating wavelength (i.e., the wavelength at which the transmission loss through the grating is a maximum, also referred to as xcexB) to changes in temperature is a minimum (e.g., the slope of the curve is zero), with the curve increasing smoothly with both increases and decreases in temperature away from the temperature Tm.
Another aspect of the invention resides in that applicants have found that by controlling the constituents of the core and cladding materials of a fiber in certain ways the characteristic temperature Tm can be made to fall at any desired temperature within a useful range of temperatures for the fiber. U.S. Pat. No. 6,201,918 shows, in FIG. 6, a grating having a Tm lying within a range of xe2x88x925xc2x0 C. to +5xc2x0 C., and an article entitled Temperature Insensitive Long-Period Fiber Grating by K. Shima, S. Okude, T. Sakai, A. Wada, and R. Yamauchi in Fujikura Technical Review 1998, pages 1 and 2, shows, in FIG. 2, a grating having a Tm lying within a range of +15xc2x0 C. to +25xc2x0 C. Neither of these publications, however, suggests the idea of intentionally controlling the constituents of the core and cladding materials to cause the Tm of a fiber, or of a grating or other optical system component made from the fiber, to fall at a wanted temperature, and especially no suggestion is made for intentionally causing the Tm of a fiber to fall at a useful temperature of the fiber not within the range of xe2x88x925xc2x0 C. to +5xc2x0 C. or the range of +15xc2x0 C. to +25xc2x0 C.
In keeping with the above, the invention resides in part in an optical fiber for use in making optical communication system components, especially long-period gratings, in said components, and in the method for making such fiber and components, wherein the core material and the cladding material of the fiber are so formulated that their characteristic curves of index vs. temperature are of dissimilar shapes and yet are such that at a selected temperature, which may be approximately the mean or average temperature at which a component made from the fiber is expected to be used, the curves of rate of change of index with respect to temperature of the core   (            i      .      e      .        ,                            ⅆ                      n            core                                    ⅆ          T                    ⁢              xe2x80x83            ⁢      v      ⁢              xe2x80x83            ⁢              s        .                  xe2x80x83                ⁢        T              )
and the cladding   (            i      .      e      .        ,                            ⅆ                      n            clad                                    ⅆ          T                    ⁢              xe2x80x83            ⁢      v      ⁢              xe2x80x83            ⁢              s        .                  xe2x80x83                ⁢        T              )
materials cross one another (and are therefore equal to one another), or at least have a minimum difference, at approximately the selected temperature; and as a result of this the rate of change of the effective index of the fiber is acceptably low or zero at the above-mentioned temperature Tm, which is equal to the selected temperature, and throughout a substantial range of temperatures extending both below and above the selected temperature.
In another aspect, the invention resides in the method of making an optical fiber with high temperature insensitivity by first making a fiber having regions of core material and cladding material which materials are formulated with the expectation that a fiber drawn from the preform may have characteristic curves of core index vs. temperature and cladding index vs. temperature of such shape that the curves representing the rate of change of these characteristic curves will cross one another or will have a minimum difference at or near the selected temperature, then testing the responsiveness of the fiber, or of a component made from the fiber, to inputs over a range of temperatures including the selected temperature, surmising from this testing how the constituents of the core material and/or of the cladding material are to be changed to have the their rate of change curves cross one another or have a minimum difference at a point closer to the selected temperature, making a new fiber in accordance with the surmise and then repeating these steps, if necessary, until acceptable formulations for the cladding and core materials are found.
Still further, another aspect of the invention resides in applicants having discovered that, for reasonable ranges of temperatures, say from xe2x88x9220xc2x0 C. to 90xc2x0 C., after having made a preliminary fiber with a certain composition of cladding constituents, such as silicon dioxide (SiO2), phosphorus pentoxide (P2O5), and fluorine(F), and a certain composition of core constituents, such as silicon dioxide, germanium dioxide (GeeO2), and boron trioxide (B2O3), and after having measured the Tm of that preliminary fiber, if a new fiber is made having substantially the same compositions of constituents for its cladding and its core as the preliminary fiber, except for the concentration of B2O3 in the core, the new fiber can be made to have a Tm higher than that of the preliminary fiber by making the new fiber with a greater concentration of B2O3 in its core than the preliminary fiber, or can be made to have a Tm lower than that of the preliminary fiber by making the new fiber with a lesser concentration of B2O3 in its core than the preliminary fiber.
In yet a further additional aspect, the invention can reside in an optical communication fiber wherein at least the core of the fiber, and also possibly the layer of cladding immediately surrounding the core, is made such that the time required for light of a given wavelength to move between two points on the core does not change significantly over a large practical temperature range.
The invention can also reside in a communication fiber of the above-mentioned temperature insensitive type wherein at least the core, and possibly also the layer immediately surrounding the core, is photosensitive in that it has an initial or base index of refraction and is subject to having its index of refraction changed from the base value at desired points along the length of the fiber by the application of actinic radiation to the fiber at the desired points.
The invention can further reside in the fiber being one wherein the core and the cladding layer immediately surrounding the core are made of silicon dioxide with the silicon dioxide of both the core and the immediately surrounding layer including dopants which provide the fiber with its photosensitive and temperature insensitive characteristics.
The invention in more detail also resides in the specific dopants and the specific concentrations of those dopants and the specific sizes of the core and the immediately surrounding cladding used in making the fiber.
In one aspect, an optical filter is provided according to the invention. The optical filter comprises an optical fiber having a core of first light conducting material and a layer of second material disposed about the core, where the first material includes 6-10 mole percent of GeO2, 1.5-3.5 mole percent of B2O3, and where the remainder mole percent- is SiO2, and where the second material includes 0.5-1.0 mole percent of P2O5, 0.4-1.0 mole percent of F, and where remainder mole percent is SiO2. In addition, the fiber includes a plurality of locations spaced along the length of the fiber wherein at least part of the fiber is thinned or wherein the index of refraction of at least part of the fiber has been changed via the application of actinic radiation.
The invention also resides in a method or methods for providing fibers such as those described above.
Other features of the invention will be apparent from the following disclosure, including the claims.